Integrating sludge drying in biomass fueled CHP plants

Energ. Ecol. Environ. https://doi.org/10.1007/s40974-020-00187-x ORIGINAL ARTICLE Integrating sludge drying in biomass fueled CHP plants Jinshan Wang1,2 • Chaudhary Awais Salman1 • Bin Wang2 • Hailong Li1 Eva Thorin1 1 2 • School of Business, Society and Engineering, Mälardalen University, 72123 Västerås, Sweden Key Laboratory of Refrigeration Technology of Tianjin, Tianjin University of Commerce, Tianjin 300134, China Received: 26 June 2020 / Revised: 23 August 2020 / Accepted: 29 August 2020 Ó The Author(s) 2020 Abstract Handling sludge through thermal conversion is environmentally friendly, which, however, requires sludge drying. This work proposed to use the waste heat of flue gas (FG) to dry sludge. The integration of sludge drying in biomass fueled combined heat and power (CHP) plants can clearly affect the performance of downstream processes in FG cleaning, such as flue gas quench (FGQ) and flue gas condenser, and further affect the energy efficiency of CHP. In order to understand the influence, a mathematical model and an Aspen PLUS model were developed to simulate the drying process and the CHP, respectively. Based on simulations, it is found that the increase of feeding rate of sludge and the moisture content of sludge after drying can decrease the water evaporation in FGQ. An increase in the feeding rate of sludge in combination with a drop of moisture content of sludge after drying can decrease the heat recovery from FG. When using dried sludge to replace biomass, the amount of saving could be influenced by the moisture content after drying and the flow rate of sludge. Simulation results show that drying sludge to a moisture content of 40% leads to the maximum biomass saving. Cd cFG cPG cpv cpw d dUsludge Keywords Flue gas quench  Heat recovery  Sludge drying  CHP  Energy efficiency List of symbols A1 Contact area between the sludge and the heated wall (m2) A2 Heat dissipation area of sludge (m2) Aw Area of heat and mass transfer per unit time in FGQ (m2/s) MH2 O Mair mDS md mFG mv PT PV PV,S(Ts) & Hailong Li Qcv Qin G g DH h hbw hc hm hrad hws Drag force coefficient Specific heat of FG (J/kg K) Specific heat of air (J/kg K) Specific heat of water vapor (J/kg K) Specific heat of liquid water (J/kg K) Diameter of droplets (m) Heat used to change the temperature of sludge bed (J) Flow rate of dry FG (kg/s) Acceleration of gravity (m/s2) Latent heat of vaporization in the surface water of sludge (J/kg) Coefficient of heat transfer in FGQ (W/m2 K) Heat transfer coefficient of static sludge (W/ m2 K) Coefficient of convective heat transfer in the surface of sludge (W/m2 K) Coefficient of mass transfer in FGQ (W/m2 K) Coefficient of radiant heat transfer in the surface of sludge (W/m2 K) Complex coefficient of heat transfer between FG and sludge (W/m2 K) Mole mass of water (g/mol) Mole mass of air (g/mol) Mass of dry sludge (kg) Mass of water droplets (kg) Mass flow rate of FG (kg/s) Drying rate (kg/m2 s) Total pressure (Pa) Partial vapor pressure of sweeping air (Pa) Partial vapor pressure of the surface of sludge (Pa) Released heat of FG in a control unit (J) Heat of FG enters the sludge bed (J) 123 J. Wang et al. Qsen Qevap Qout Tbed,i TFG TG Ts Tw ud ug X ys Sensible heat transferred into sweeping air (J) Latent heat transferred into sweeping air (J) Rest heat transferred into the sweeping air from sludge bed (J) Initial temperature of sludge in the one contact period (°C) FG temperature in dryer (°C) Sweeping air temperature (°C) Surface temperature of sludge (°C) Water droplet temperature (°C) Velocity of droplet (m/s) Velocity of the FG before FGQ (m/s) Moisture content of sludge (kg/kg) Saturated humidity at droplet surface (kg/kg) Symbols k Heat conductivity coefficient of FG (W/m K) kq Heat of vaporization in FGQ (J/kg) qg FG density (kg/m3) 1 Introduction With the unceasing growth of wastewater, the amount of sludge increases rapidly (Kor-Bicakci et al. 2019; Zheng et al. 2020), which has become one of the most severe environmental problems around the world. The conventional methods of sludge management are through landfilling or agricultural applications. However, the contents of heavy metals, organic pollutants and pharmaceuticals result in a high risk of secondary pollution and therefore, they might no longer be viable due to more strict regulations and the rising environmental and health concerns (Kim et al. 2019; Wang et al. 2019a). For example, the requirements of European directives 99/31/EU already indicate that landfilling of sludge is not a desirable option (The council of the European Union 1999). Instead of being buried directly, sludge can be handled through thermal conversion, such as: pyrolysis, gasification and incineration. Pyrolysis is regarded as environmentally friendly technology, in which sludge could be converted to bio-oil and bio-char. However, drying is usually needed due to the high moisture content (Kuan et al. 2020). Gasification occurs at a higher temperature. The advantages of sludge gasification include complete sterilization of sludge and large mass reduction (Lee et al. 2013). Nevertheless, drying is also demanded to a moisture content lower than 25% (Ayol et al. 2019). Sludge incineration is attracting more interest (Murakami et al. 2009), which can significantly reduce sludge volume, eliminate odor and stabilize sludge (Chen et al. 2017). Similar to pyrolysis and gasification, wet sludge cannot be incinerated directly. Moreover, the high moisture content can also affect the 123 other performances of sludge incineration, including pollutant emission through both flue gas (FG) and wastewater, combustion efficiency and energy recovery. Many works have been done about sludge drying (Ameri et al. 2020). Usually, dryers can be divided into direct drying, indirect drying and hybrid drying (or mixed drying). For direct drying, heat medium passes through sludge and water is vaporized. Examples include direct heating drum dryer (Farid et al. 2019), flash dryer and belt dryer (Tańczuk et al. 2016). Hot-air (heating by the electrical heater), steam and FG are commonly used as the heat medium. Direct drying has the advantages of easy manipulation, but it has the relatively long drying time, bad odors and gaseous emissions (Léonard et al. 2008; Arlabosse et al. 2011; Fraikin et al. 2011). The specific energy consumption is ranged from 700 to 1400 kW h/t, and the specific drying rate varies from 0.2 to 30 kg/m2 h (Bennamoun et al. 2013). For indirect drying, the sludge is heated through a heat exchanger, for instance the rotary dryer, vertical multi-tray dryer and paddle dryer (Schnell et al. 2020; Charlou et al. 2015). Indirect drying can avoid the pollution of the heat carrying medium and reduce the risks of fire and explosion. The volatile organic compounds (VOC) concentration is low, and the steam and odor is confined (Ferrasse et al. 2002). But during indirect drying, the sludge exepriences a p (...truncated)